U.S. Pat. No. 4,207,441 discloses an auditory prosthesis equipment comprising n electrode sets implanted in the cochlea at n discrete locations chosen to allow the brain to identify n discrete frequencies in the sound range. Signals collected by a microphone are passed through a compressor to adapt the dynamic range of the sound information (e.g. 60 dB) to the dynamic characteristics of the ear (e.g. 4 dB). An analysing network transforms the compressed signal into n analysis signals using filters with frequencies corresponding to the frequencies identifiable by the brain. An amplitude calculating network calculates the mean value of the amplitudes of each analysis signal, and a signal forming network generates n pulse signals in dependence on the analysis signals. A logic circuit provides a raster signal with width-modulated pulses in dependence on the n mean values, the n pulse signals and a matching network for preadjustment to individual users. The raster signal is transmitted inductively to an implanted receiver, which sequentially transmits electric pulses to respective electrode sets in the n electrode sets such that the energy transmitted in each electric pulse corresponds to the energy indicated in the respective pulse in the raster signal.
The compression of the microphone signals in the disclosed auditory prosthesis equipment inherently changes the frequency composition in the signals, and thus creates artefacts in the n analysis signals provided by the subsequent frequency filtering. Furthermore, the signal processing in the disclosed auditory prosthesis equipment does not provide any specific enhancement of speech signals compared to other acoustic signals.